1. Field of Invention
The present invention relates to co-extrudable adhesive compositions suitable for composite structures of barrier layers and structural layers, and specifically for ionomer heat-seal structural layers. More particularly, it relates to co-extrudable adhesives based on a blend of two polyethylenes one of which is very low density polyethylene, blended with a hydrocarbon elastomer. All components may optionally be acid grafted. Such adhesives provide bonds to ionomers which are both strong, and which do not `age-down`.
2. Description of Related Art
Commercial packaging applications often employ composite structures of multiple layers of thermoplastic polymers. These composite structures typically include barrier layers for water, oxygen or other substances, and structural layers bound together by an adhesive or `tie layer`. The inner structural layer may also serve as a heat-seal layer allowing ready bonding to itself. The barrier layers may be polar materials particularly suitable for oxygen barriers such as ethylene/vinyl alcohol copolymer or nylon, or non-polar water barriers such as all hydrocarbon polyolefins. Heat-seal layers are commonly ethylene copolymers. Ionomers, particularly sodium ionomers, have particularly desirable heat-seal properties.
Composite structures are often made by extrusion processes. In such processes, the adhesive is applied to the structural layers in the molten state. Commonly, one or more structural layers is also extruded in the molten state. When adjacent layers are simultaneously extruded from the melt, the process is known as co-extrusion, and adhesives useful for the process as co-extrudable adhesives.
Co-extrudable adhesives which are useful for bonding barrier polymer layers are well known. Polar polymers such as ethylene/vinyl acetate copolymers can be used as adhesives to bond many polymers to ionomer. However adhesives based on acid modified polyethylene are superior in organoleptic properties, in applications requiring exposure to elevated temperatures, and are generally superior in bond strength, to polar barrier polymers such as ethylene/vinyl alcohol. Such acid modified polyethylene based co-extrudable adhesives may also be toughened with elastomers.
Even so, such co-extrudable adhesives tend to be problematical when used to bond barrier layers to ionomer because, while they bond to barrier resins such as ethylene/vinyl alcohol, they tend either to provide poor bonds to ionomer or to produce bonds that, while strong initially, tend to drop off rapidly with time (age-down) or drop off in aggressive environments. This is particularly true when are ionomer is the commonly used sodium ionomer.
U.S. Pat. No. 4,198,327 (Matsumoto et al.) discloses a composition having improved adhesion to solid polar materials which comprises a modified crystalline polyolefin having an X-ray crystallinity of at least 25%, and not more than 10 mole % comonomer, the polyolefin having grafted thereto a monomer such as an unsaturated carboxylic acid or it derivative, the composition additionally containing a hydrocarbon elastomer. The elastomer is disclosed as necessary to provide initial and durable adhesion, and works only when the polyethylene is acid grafted. Among the extensive number of polar materials listed, ionomer resins are not included. No very low density polyethylene having less than 25% X-ray crystallinity is included in the adhesive. The elastomers appear to be uncured.
U.S. Pat. No. 3,868,433 (Bartz et al.) discloses polyolefins generally, graft modified with acids, and which may also contain elastomers, for use as hot-melt adhesives.
U.S. Pat. No. 4,684,576 (Tabor et al.) discloses adhesive blends based on grafted high density polyethylene of density 0.94 to 0.965 containing linear low density polyethylene of density 0.88 to 0.935, for use as adhesives. No elastomer is present. The adhesive of the present invention can not contain any high density polyethylene.
European Patent Publication No. EP 0,495,996 A1 also discloses a blend of a high density polyethylene of density greater than 0.935 which may be acid grafted, and a linear low density polyethylene which may have a density of either 0.910 to 0.935 which may also be grafted, or a density of 0.890 to 0.910 (referred to also as a linear very low density polyethylene), or both, which has good adhesive behavior when used with ethylene/vinyl alcohol, polyesters, and certain metals.
European Patent Publication No. EP 0. 479,457 also discloses a polyolefin adhesive composition comprising a high density polyolefin of density 0.935 or higher grafted with acid, and containing a very low density polyethylene of density 0.86 to 0.915. The composition may contain, but is preferably free of hydrocarbon `rubbers`. Adhesivity to steel, and to polar substrates, especially polyamides and ethylene/vinyl alcohol polymers is disclosed. The present invention can not contain polyethylenes having a density greater than 0.935.
There is a need for thermoplastic co-extrudable adhesives which have the specific ability to bond to ionomers with a high level of permanence in the bonding, to allow ionomers to be used more readily as heat-seal layers in durable multilayer packaging films.